Analysis of low-frequency impedance processes in segmented PEM fuel cells

Under technically relevant air-fed polymer electrolyte membrane fuel cell (PEMFC) operation, the low-frequency impedance response is shaped by overlapping oxygen-transport processes along the cathode channel and through the porous gas diffusion layer (GDL), microporous layer (MPL), and cathode catalyst layer (CCL). Local ionomer hydration and liquid-water accumulation further modify these transport processes. In this study, these low-
frequency contributions are resolved locally using an along-the-channel segmented PEMFC by combining sequential segment-wise electrochemical impedance spectroscopy (EIS) with subsequent distribution of relaxation times (DRT) analysis. Targeted variations in inert gas, cathode stoichiometry, current density, and relative humidity were evaluated together with along-the-channel current-density distributions, allowing two distinct low-frequency processes, P$_{LF1}$ and P$_{LF2}$, to be separated over the investigated operating window. The results show that the relaxation frequency of the lower low-frequency peak, PLF$_{LF1}$, depends primarily on channel-gas velocity. Its polarization contribution is additionally influenced by oxygen depletion and oxygen transport along the channel. ... mehrThe higher low-frequency peak, P$_{LF2}$, is governed mainly by local through-plane oxygen transport through the GDL, MPL, and CCL and responds strongly to the effective oxygen diffusion coefficient, local oxygen availability, local current density, and water-modified transport conditions in the porous cathode. These findings establish a mechanistic distinction between a predominantly channel-related low-frequency contribution and a predominantly local through-plane oxygen-transport contribution in the porous cathode, providing a physically informed basis for interpreting low-frequency impedance signatures under technically relevant PEMFC operating conditions.